Over the last 10 years numerous epidemiologic studies have found increase morbidity and mortality associated with particulate air pollution. Much of the excess risk is associated with cardiovascular disease events and concern has been raised as to the potential mechanisms whereby such effects can be explained. In addition the effects to date have been associated with total particle loads expressed in terms of mass. These data have led to toxicologic investigatiolls that have raised several hypotheses related to autonomic nervous system control over cardiac function and how such control might relate to an excess of sudden death and or other cardiovascular events. We propose to identify and physiologically characterize potentially high risk free-living patients and monitor them repeatedly for 48 hours for heart rate, heart rate variability, ST segment and T-wave changes in association with detailed outdoor and indoor particle monitoring. Respiratory variation will be taken into account in overnight studies and activity diaries will be used during the day. We postulate that evidence of electrophysiologic changes will be associated with particulate load and will be changes that have been associated with risk of myocardial infarction~ and other cardiovascular events. These findings may help to better identify mechanisms of injury associated with the reported excess morbidity and mortality and may allow for the identification of particle characteristics associated with these outcomes and development of suggestions for preventive strategies.